Reorientational dynamics and the barrier to internal rotation in Cp2Co3(.mu.2-CO)(CO)3(.mu.3-CPh)

Abstract

NMR spin-lattice relaxation times of the phenyl, cyclopentadienyl, and terminal carbonyl 13 C nuclei in the title compound were measured as a function of temperature in the solvent chloroform. The phenyl carbon T 1 's were used to calculate the diffusion coefficient characterizing the ring's net spinning rate. Internal rotation rates of the cyclopentadienyl groups, R(Cp), were found to be far greater than R(Ph) at all temperatures, indicating that the barrier to phenyl group rotation is not due to inter-ring steric repulsions. Rather, it can be attributed to an electronic interaction of the bridging carbonyl's π* orbital with the phenyl π system through the metal 2e and carbynyl 2p orbitals. This is only the second system in which a bridging carbonyl/capping group orbital interaction has been observed and the first for which the barrier range to internal rotation has been experimentally determined